Since you are just failing, do the standard tune-up, also pull the cat and see how it looks, you should be able to see light through it. If the cat looks good, the tune-up might be all you need. Also make sure the engine is good and hot, if you live in town take it out onto teh highway for a few miles, and try to get the shop to check the truck when you get there, don't let it idle while you wait.

You can also try running a higher octane gas, mixing in a gallon of denatured alcohol to 10 gals of gas, a new cat will knock the numbers down(unless your cat is already new). These are the cheapest approaches. If none of them work, you will need to adjust the carb, though a 5 CO higher is doubtful that it is the carb.